[SR-3927] Lockdown the layout of value witness tables

[ematejska]

Previous ID	SR-3927
Radar	rdar://problem/31411195
Original Reporter	@ematejska
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AffectsABI
Assignee	None
Priority	Medium

md5: 48a329510b6da2be90b89229c10f7f72

Issue Description:

Ahead of ABI Stability, lockdown the value witness table entries and structure.

[bob-wilson]

@swift-ci create

[bob-wilson]

We have made some changes for value witness tables:

• Removed array operations (https://bugs.swift.org/browse/SR-3376)

• Added value witnesses for single payload enums (https://bugs.swift.org/browse/SR-4332)

Beyond those, we do not plan for other changes in value witness tables.